In this chapter, the probabilistic porous-solid model is used to determine the mean effective stress of soils at failure. The plots of the deviator stress against the mean effective stress show a unique failure line for a series of triaxial tests performed at different confining net stress and suctions for both wetting and drying paths. This result confirms that the proposed effective stress equation is adequate to predict the shear strength of unsaturated soils. It also results in different strengths for wetting and drying paths, as the experimental evidence indicates.